At the present time, there are three known methods for reducing nitro groups to amine groups.
The first method is generally referred to as the chemical reduction and involves carrying out the reduction of the nitro groups to amine groups by treatment of the aromatic nitro compounds with metals such as zinc, tin or iron in the presence of an acid such as hydrochloric acid. On the other hand, when the reduction is carried out with zinc in a neutral or basic aqueous medium, the aromatic nitro compound is reduced to hydroxylamines or azo compounds respectively. It is also known that lithium aluminum hydride reduces aromatic nitro compounds to amines whereas borane is known to reduce aliphatic nitro compounds to hydroxylamines.
Another form of reduction is that carried out by catalytic hydrogenation whereby aromatic and aliphatic nitro compounds are reduced to amines with hydrogen in the presence of Raney nickel, platinum or palladium usually in finely divided form. Raney nickel is usually formed `in situ` while platinum and palladium are used as the oxides which are reduced in the course of the hydrogenation to the elementary metals, or as metals supported on charcoal, calcium carbonate or other inert carrier.
Finally, there is an extensive literature on electrochemical reduction of aromatic nitro compounds to amines, p-aminophenols, azoxy compounds, azo compounds, hydrazo compounds and benzidines in aqueous or aqueous-alcoholic solutions, the course of the reaction depending upon the pH of the solution. It is also known to reduce aliphatic nitro compounds to hydroxylamines in an acidic medium.
Notwithstanding the respective advantages of each of the above-described reduction methods, none of them displays simultaneously the advantages of high selectivity, low cost, safeness, absence of pollution and toxicity.
Accordingly, it would appear to be highly desirable if a hydrogenation electrode could selectively hydrogenate nitro compounds to the corresponding amines even in the presence of other functional groups while providing highly desirable yields and current efficiencies.